Small vehicles such as lawn and garden tractors are commonly equipped with hydrostatic transmissions. The hydrostatic pump-motor circuits in these transmissions generally remain pressurized when the engine is not operating. Consequently, movement of the unpowered vehicle when routine maintenance and/or other repairs are to be performed is difficult since fluid flow through the pump is not possible without a line being provided to bypass the pump.
While it is common for lawn and garden-type vehicles with hydrostatic transmissions to provide hydrostatic fluid pressure release circuits and valve controls that allow the operator to manually move the vehicle for maintenance and other activities when the engine is not operating, the controls for opening and closing those circuits present problems.
One such vehicle, identified as the John Deere Model 300/400 Series Lawn and Garden Tractor, provides a hydrostatic fluid pressure release lever which is set and then locked in place by the operator. Should the operator fail to return the lever to its original position before restarting the engine, the hydrostatic fluid release valve will not permit the transmission to build up pressure, and accordingly, the transmission will have no forward or reverse speed. Once the engine is started and the pressure release control lever is returned to its original position, the tractor may lurch as hydrostatic pressure is restored.
A second type of hydrostatic fluid pressure release control is found on the 200 Series John Deere Model Lawn and Garden Tractors. Once the pressure release lever is activated on this vehicle, it remains in its released position. When the engine is subsequently started, the pump pressure in the hydrostatic transmission closes the pressure release valve, and the transmission functions normally. This type of release control system however requires a high charge pump pressure to close the hydrostatic fluid pressure release valve automatically and, of course, adds additional manufacturing and maintenance costs to the vehicle.
A third type of hydrostatic fluid pressure release control mechanism is found in the 100 Series John Deere Model Tractors. With this control, the operator depresses a fluid pressure release valve lever beneath his seat to release the fluid pressure. The lever has a spring return and therefore must be held in the released position during the entire time the tractor is being pushed. While this system overcomes the need for the operator to release the lever after manually moving the vehicle, it can be inconvenient because the operator must simultaneously hold the release lever in place, push the tractor and steer it at the same time.